The proposed research will investigate whether 5-month-old infants can perceive the numerosity of temporal sequences of elements (flashes of light and tones), and whether infants can perceive equivalences and differences of numerosities across sensory modalities. The ability to perceive small (N less than or equal to 4 or 5) numerosities (subitizing) is a well-established phenomenon in adults and children. The age at which it appears and the role it plays in the development of later number skills is the subject of current theoretical dispute. An habituation procedure will be employed in which infants, habitutated to stimuli of varying durations but equal numerosity (N), will next be presented a stimulus with the same total and inter-element durations as 50% of the habituation stimuli but a different numersotiy (e.g., N plus 1). If response recovery (increased heart rate deceleration and negative skin potential amplitude) occurs, then numerosity perception can be inferred. A magnitude-of-response-recovery score will be derived to infer cross-modal perception of number. Evidence for numerosity perception would disconfirm one class and support another class of number development theories. Evidence for cross-modal perception of numerosity would indicate that infants possess a central, amodal estimation process for perceiving small numerosities.